This invention relates to communications systems, multicarrier modulation systems, orthogonal frequency division multiplexing (OFDM), and specifically to synchronization in an OFDM system.
OFDM principles have been known in the art for many years. In recent years, OFDM has been applied to broadcasting in such systems as the European DAB (digital audio broadcasting) standard, to high definition television (HDTV) and to communications systems for military and civilian applications requiring high digital data rates over narrow bandwidths. OFDM is the modulation format included in IEEE Standard 802.11b.
OFDM uses multiple orthogonal subcarriers with minimal subcarrier spacing to convey information over multiple subchannels. Sequential streams of data are transmitted simultaneously on each subcarrier and at any instant in time many data symbols are being transmitted. A high data rate stream may be broken into many low data rate streams and transmitted over an OFDM system. In a multiuser system, many users may each use one or more of the subchannels. The bandwidth of every individual data stream occupies a small fraction of the available bandwidth. By transmitting data simultaneously on many low-rate subchannels, a wideband transmission system is converted Into many narrow-band systems. To obtain high spectral efficiency, the frequency spectrums of the subcarriers partially overlap with specific orthogonality requirements to enable separation of the subcarriers at a receiver. The larger the number of subcarriers, N, the longer the symbol period becomes making the system less susceptible to burst errors and delay spread. The number of subcarriers N, however, is in practice limited by the filtering process,
computational time, the available transmission bandwidth of the channel, and the Doppler shift.
Orthogonality between the subcarriers can be maintained, even if the signal is passed through a time-dispersive channel, by adding a cyclic prefix or extension at the beginning of every OFDM symbol. The cyclic extension is a copy of the last part of the OFDM symbol of length equal to or greater than the maximum delay spread of the channel. Insertion of the cyclic extension imposes a penalty in terms of transmitted power and available bandwidth but improves symbol timing to reduce intersymbol interference (ISI).
In a transmitter in an OFDM system, a serial input data stream is converted to parallel data. Forward error correction may used on the data stream. The parallel data stream is then applied to a signal mapper to set the amplitude and phase of each subcarrier in the form of complex values according to a predetermined modulation constellation. Such modulation formats as quadrature amplitude modulation (QAM) and quadrature phase shift keying (QPSK) may be used. An inverse fast Fourier transform (IFFT) converts the frequency-domain phase and amplitude data for each subcarrier into a block of N time domain samples. The samples are combined together and the cyclic extension is added. The resulting time domain samples are then converted to an analog modulating signal that is then input to a RF modulator and transmitted. The reverse process is implemented in an OFDM receiver. An FFT is used to extract the phase and amplitude of each received subcarrier from the block of received samples.
Synchronization is required between the transmitter and the receiver for the receiver to recover the data. Synchronization is required to correct for frequency offsets between oscillators in the transmitter and receiver. Such frequency offsets cause loss of orthogonality leading to intercarrier interference (ICI). Symbol synchronization is also required at the receiver to know where a data symbol starts. A timing offset results in phase rotation of the subcarriers. Use of the cyclic extension reduces the timing error problem.
Communications systems for military applications place several constraints on the over-the-air waveform utilized. The waveform should have a low probability of intercept and detection (LPI/LPD) and be resistant to jamming threats.
Traditional OFDM systems use the cyclic extension or a zero extension between bursts for multipath mitigation and timing synchronization. This cyclic extension, however, provides a significant feature to the transmitted waveform. A synchronization pattern may be used on some of the subchannels in an OFDM system. The synchronization pattern traditionally uses a specific set of tones in an OFDM symbol in the transmitter to generate a time domain sequence. The time domain sequence has repeating patterns that are used in the receiver to extract time and frequency information. This obviously contains a feature that can be exploited by an adversary. As an option to this, some systems use single carrier synchronization symbols to obtain time and frequency information.
A featureless symbol buffer in place of the cyclic extension or zero extension maintains the multipath mitigation properties of the OFDM waveform while reducing the delectability. Simulation and analysis show that a small cyclic extension with the remainder of the burst being random In nature IS not easily detectable and yet provides desirable attributes to the transmitted waveform.
A synchronization method is required that will not interfere with other users sharing the same channel bandwidth while at the same time not providing any easily detectable features.